Indian Journal of Chemical Technology
Vol. 8, July 2001, pp. 255-259
A method of separation of lime from high or low calcium oxide
containing limestone
S C Sarker
Analytical Laboratory, DMR Risa Colony, Shillong 793 003, India
Received 11 May 2000; accepted 20 March 2001
A method of separation of lime (CaO) from limestone either by treating with water or by treating with concentrated
hydrochloric acid at pH 9-10. This method will help to enrich the concentration of various other elements associated with
limestone and pure CaO may be used for industrial purpose.
The interest in limestone is due to its accelerated and
diverse uses. It is the one mineral for which over 2000
uses are known 1. In nature limestone is widely scattered either as simple mineral calcite or associated
with other minerals. There are several books 1·3
which cover the details of commercial grading,
exploitation and industrial utilisation of limestone. Literature survey revealed that in chemical industry, limestone is mostly used in the form of lime
(CaO), a major intermediary product of calcined limestone1.
Reported chemical analysis of limestone2•3 and
dolomitised limestone2 shows that limestone is
generally associated with Si0 2, MgO, Ah0 3, Mn0 2,
Ti02, Fe20 3, Na20, K20, etc. as impurities and often
Uses
of
display
considerable
variability.
limestone/lime depend upon its purity. But
comparatively little work has been reported on the
separation of lime in pure form from limestone. Some
of earlier reports describe the preparation of calcium
compound from limestone viz.(i) CaO prepared by
calcining limestone4 • (ii) CaCh is prepared by the
reaction of hydrochloric acid and limestone, where
FeCh obtained as impurity which is removed by
chemical treatment4 • (iii) From dolomite, calcium is
removed by ion-exchange after calcination5 • (iv)
Mg(OHh precipitated from MgClz or seawater by
milk of lime4•6 · 8 . In most of the cases high calcium
oxide containing limestone is used because of its
availability. So the demand for low calcium oxide
containing limestone is comparatively less at present,
but may be very feasible and attractive resource.
The present work describes a method of separation
of lime from the associated oxide containing
limestone.
Experimental Procedure
Limestone samples (seven numbers) were selected
having different chemical composition. The sample is
used in powder form and dried in an air-oven at
l05°C. The percentage of various oxides present in
limestone, burned limestone and oxides remain after
separation of CaO from burned limestone was analysed by standard methods 12 . A solution was made by
taking a known weight (5g) of sample, digested with
hydrochloric acid (1: 1) and few drops of concentrated
nitric acid, baked (120°C), dissolved in dilute hydrochloric acid and filter. Silica was determined by hydrofluorization of the insoluble left and percentage of
silica was calculated as,
where w2 = weight (g) of platinum crucible before
hydrofluorization.
w 3 = weight (g) of platinum crucible after hydrofluorization with the residue.
w1= weight (g) of the sample.
The residue was fused after hydrofluorization with
fusion mixture and extracted with hydrochloric acid
( 1: 1). Extract was added to the filtrate obtained before
and made up to the known volume (100 mL). From an
aliquot (25 mL) of the main solution, mixed oxide
(R 20 3) was precipitated by ammonia solution in the
presence of ammonium chloride, filtered, ignited and
weight as R20 3 is calculated as follows.
R203%
= {(wz-W3J x 100}/w1
where w2 = weight (g) of the silica crucible with R20 3
after ignition.
INDIAN J. CHEM. TECHNOL., JULY 2001
256
Table !-Analytical data
MgO
a
39.30
66.95
42.12
3.30
5.71
10.75
4.80
8.22
15.64
9.64
16.35
31.45
29.58
49.70
23.07
4.22
6.98
11.51
8.24
13.90
24.45
15.71
26.26
42.90
c
51.30
88.26
59.98
1.10
2.00
8.30
2.56
4.50
19.33
1.64
2.80
12.15
4
a
b
c
52.44
91.92
37.08
0.10
0.18
1.90
1.36
2.40
25.63
1.86
3.21
35.24
5
a
32.24
48.50
21.76
16.20
24.55
38.73
9.04
13.59
21.50
7.29
11.48
18.01
29.58
49.70
15.33
4.22
6.98
12.67
8.24
13.90
24.76
15.71
26.26
47.22
b
d
39.30
66.95
19.47
3.30
5.71
14.98
4.80
8.22
21.75
9.64
16.35
43.70
8
a
b
d
51.30
88.26
48.82
1.10
2.00
10.98
2.56
4.50
24.72
1.64
2.80
15.53
9
a
b
53.71
92.65
43.30
1.05
1.83
21.99
0.96
1.67
20.17
0.69
1.20
14.42
52.44
91.92
25.95
0.10
0.18
2.19
1.36
2.40
30.28
1.86
3.21
41.47
52.56
91.72
31.54
1.13
2.00
24.21
1.28
2.26
27.38
0.79
1.40
16.80
b
c
2
a
b
c
3
a
b
b
d
6
a
b
d
7
a
d
10
a
b
d
11
a
b
d
aPercentage of various oxide in original limestone sample.
bPercentage of various oxides estimated in experimental procedure (A) after burning the original limestone sample, a.
c.& dPercentage of various oxides remain in experimental procedure (B) and (D). data in line care treated with water (4 times for
sample 1,2 and 6 times for sample 3, 4) and d treated with hydrochloric acid of the burnt limestone, b. R 2 0r-~Fe 2 03 , Al 20 3, etc
w3
=weight (g) of the empty silica crucible.
w 1 = weight (g) of sample.
After removing mixed oxides (R 20 3 ) from known
aliquot of the main solution, the filtrate was acidified
with hydrochloric acid and calcium was precipitated
as calcium oxalate. The precipitate was filtered,
washed, dissolved in hot sulphuric acid (1 :4), and
warm solution was titrated against standard potassium
permanganate solution and CaO % was calculated as
follows.
CaO%
={AxBxO. 028x100}/C
where A = volume of standard potassium permanganate solution used.
B = normality of potassium permanganate solution.
C = weight (g) of the sample representing aliquot
taken .
For MgO, removed mixed oxides from an aliquot
(10 mL) of the main solution as mentioned earlier and
titrated against standard ethylenediaminetetra-acetic
acid disodium salt for percentage of total (CaO +
MgO) in the presence of buffer solution and percentage of MgO was calculated as
MgO % =(total %-CaO %)11.4.
Analytical and experimental data are presented as
Tables 1 and 2. An electric muffle furnace (-1200°C
max) and electronic balance, Shimadzu, AEU220
were used for carrying out the experiment. pH were
measured by Whatman pH paper. pH range was 1-14.
All other chemicals were of analytical grade.
Calcination of limestone
(i) Dry powder limestone sample (-lOg) was taken
in platinum dish and burnt in electric muffle furnace
at 925°C for 2h.The platinum dish and content was
cooled in a desiccator and weighed. The operation
was repeated till attained constant weight and percentage of various oxides were calculated.
(ii) The burnt limestone thus obtained in operation
A is quantitatively transferred m 2000 mL glass
beaker and 1000mL distilled water is added. The solution is allowed to stand for an hour and the pH
(-14) of the solution is measured by pH paper. The
whole mixture is stirred and kept overnight. The pH
of the mother liquor is measured and filtered by
Whatman filter paper No. 40. The residue is washed
three times by distilled water, dried in air and then
transferred to platinum crucible which is burned in
electric muffle furnace at 925°C for 2h. The platinum
crucible is cooled and weighed. The operation is repeated till constant weight is obtained.
SARKER: SEPARATION OF LIME FROM CALCIUM OXIDE CONTAINING LIMESTONE
257
Table 2-Experimental data
Purity of
CaO
CaO
remaining
CaO
obtained
pH
CaO
separated
b
c
66.95
42.12(22.37)*
44.90
(44.58)
10-12
67.06
2
b
c
49.70
23.07(13.99)*
35.60
35.71
10-12
71.75
3
b
c
88.26
59.98( 14.45)*
74.51
(73 .81)
10-12
84.53
4
b
c
91.92
37.08(3.51)*
88.14
(88.41)
5
b
d
48.50
21.76(13.79)*
34.51
(34.71)
10-12
71.15
99.88
6
b
d
49.70
15.33(8.45)*
41.04
(41.25)
10-12
82.80
99 .88
7
b
d
66.95
19.47(7.42)*
60.10
(59.53)
9-10
89.77
99.88
8
b
d
88.26
48 .82(8 .89)*
80.00
(79.37)
9-10
90.64
99.90
9
b
d
92.65
43.30(3.60)*
89.40
(89.05)
9-10
96.49
99.83
10
b
d
91.92
25.95(2.13)*
89.72
(89.79)
9-10
97.60
99.90
II
b
d
91.72
31 .54(2.60)*
89.36
(89.12)
9-10
97.74
99.86
95.88
CaO remaining:-- Percentage of CaO remaining in residual oxide of experimental procedure (A), (B) & (D), represented by the data b, c
& d. of Table I.
( )* :-- equivalent amount of CaO present in b = CaO % in c or dXSi0 2 in b/Si0 2 % of c or d, value of CaO % in c or d & Si0 2 % in b, c
or d are taken from Table I.
CaO obtained :-- percentage of CaO obtained from the filtrate of experimental procedure (C) & (E), by oxalate method data in parentheses is required= CaO% in b-equivalent CaO in b.
The residue thus remaining in platinum crucible is
transferred to 2000 mL beaker and whole operation
was repeated as above till the pH of the solution
reaches 10-12. The percentage of various oxides
remaining in the residue is calculated after final
operation.
(iii) The filtrate and washing of every operation B
is collected after neutralisation by HCl and concentrated (-200 mL) on a hot plate. After concentration
CaO is precipitated from the concentrated solution by
standard oxalate method 9. The oxalate thus obtained is
then burned in platinum crucible at 925°C for 2 h in
electric muffle furnace. The crucible is cooled and
weighed. The operation is repeated till the weight becomes constant and the percentage of calcium oxide is
calculated.
(iv) Burned limestone obtained in operation A is
quantitatively transferred in 400 mL beaker and
200mL distilled water added. The solution is allowed
to stand for an hour and the pH (-14) is measured by
pH paper. The whole mixture is stirred by glass rod
and concentrated HCl is added drop-wise with stirring
till the pH reaches 9-10. The solution is kept overnight, the pH of the solution is measured and then
filtered by filter paper No. 40. The residue is washed
by cold distilled water, dried in air and transferred in
INDIAN J. CHEM. TECHNOL., JULY 2001
258
platinum crucible which is burned in electric muffle
furnace at 925°C for 2 h and then cooled and weighed.
The operation is repeated till constant weight is obtained. The percentage of various oxides remaining in
residue are calculated.
(v) CaO, is obtained from the filtrate and washing
obtained in operation D by procedure similar to operation C. CaO can be obtained from the filtrate by
treating it first with supersaturated aqueous solution
of Na 2C0 3 and then ignited at 925°C for 2 h in electric muffle furnace.
Results and Discussion
The percentage of various oxides present in the
limestone have been assigned by chemical analysis of
limestone, burned limestone and the oxides remaining
after separation of CaO from burned limestone and
are presented in the Tables 1 and 2 respectively. For
the present experiment, seven limestone samples
having CaO ranging from 53.7% to 29.58% and MgO
from 0.69 to 15 .71% were chosen.
Lime 1•8 was produced by burning limestone at well
defined temperature ranging between 900 and 1100°C.
In the present experiment limestone was burned at
925°C for 2 h in electric muffle furnace, when along
with major intermediatory product CaO, a host of
other product viz. Si02, Rz03, (Fez03 +Ah0 3 etc),
MgO were obtained. When limestone is heated at particular temperature, they decomposed evolving C02
and leaving behind the corresponding oxides 10 • In
case of dolomatic limestone 10 , magnesium carbonate
decompose first, followed by calcium carbonate e.g.
CaC 0 3.MgC 0 3
MgO+CaC03
600-SOOOC
Jllo
M g0 + c aco3 + co2
MgO + CaO +COz
Burned limestone thus obtained when treated with
water lime is hydrolysed 11 with the evolution of heat
and the hydroxide was dissolved sparingly. (1.29g of
CaO/L at l5°C or 0.67 g at 80°C)4 • Lime is not attacked by C02 at ordinary temperature but it reacts
with Ca (OHh. The burned MgO is relatively inert, it
slowly combines with water8 and forms hydroxide,
4
which is insoluble in water (lx10 g/L at 20°C) 5 .
Other oxides are insoluble or sparingly soluble in
water.
Solubility
product''
of
Ca(OHh,
Mg(OH)z,Fe(OH)J, and Al(OHh are 4x10·6, 1x10· 11 ,
4xl040 , 1x10-33 respectively at 25°C. More solubility
of Ca(OHh than that of Mg(OH) 2 favoured the separation of lime from burned limestone by treating with
water. When burned lime is treated with water, the pH
of the solution reaches 14. This may be due to the
presence of Ca(OH)z, which is a strong base, sparingly soluble in water resulting in the increased number of (OH)- in the solution compared to Mg(OHh, a
weak base. These may further reduce the solubility of
Mg(OH) 2 in the solution resulting in separation of
above 99% pure CaO, as shown in Table 2. Literature
survey further reveal that Mg(OHh can be precipitated from a solution of MgCl 24 ,or sea water by adding milk of lime6· 8 • The reduction of CaO%, increase
the other components in burned limestone. The
change in pH from 14 to 10-12 may be due to stepwise separation of CaO in the form of Ca(OHh by
repeated operation of the same sample. In the present
experiment 4 to 6 operations were carried out to reach
the pH 10-12, depending upon the percentage of CaO
in burned limestone. By this process 67 to 95% CaO
separated from burned limestone. Less yield also may
be due to the fact that Ca(OHh solution on exposure
to air becomes covered with a crust of calcium carbonate which on breaking settle at the bottom with
other components and another layer of crust reappears
at the surface. The yield can be increased from 85 to
95% by using concentrated hydrochloric acid. In this
experiment burned limestone is treated with water and
then the pH of the mixture is brought down to 9-10 by
dropwise addition of concentrated hydrochloric acid,
which react with soluble hydroxide resulting in more
and more CaO passing into the solution as Ca(OHh
and due to the continuous neutralisation by hydrochloric acid, pH of the solution comes down thereby
increasing the yield of CaO%. CaO is separated from
filtrate by standard oxalate method9 or by carbonate
method and there after by burning the oxalate or carbonate.
This observation is drawn from chemical analysis
results shown in Tables 1 and 2. By lowering the pH
below 9-10 impurities like MgO, Fe20 3, A}z03 , etc
may come along with the product thereby lowering
the purity of the yield.
This method may be used to enrich the concentration of various other elements associated with burned
limestone as shown in Table 1. The enrichment thus
obtained may help to a great extent in analytical work
on one hand and may be used as an attractive source
of some elements on the other. The pure CaO/CaC03
may be utilised for industrial purposes.
Acknowledgements
The author is thankful to the Directorate of Mineral
Resources, Shillong for permission to publish the pa-
SARKER: SEPARATION OF LIME FROM CALCIUM OXIDE CONTAINING LIMESTONE
per and all the officers and staff of the Laboratory
section for their help and suggestions.
7
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